One of the most expensive components in any optical communications system is the optical transmitter module which may, for example, be comprised of a continuous wave (CW) distributed feedback (DFB) laser and an external Mach-Zender (MZ) intensity modulator, plus associated driving and stabilization electronics. For optical communication systems, it is natural to use optical components as much as possible if conditions permit. Traditionally, the alignment of semiconductor optical devices to one another has been carried out using conventional optics. The precise alignment of the DFB laser diode to an external MZ modulator may, for example, employ a pair of aspherical lenses with an optical isolator placed in between. An optical isolator is a device used to prevent light in a system from reflecting back to the source which could deteriorate its operation. As is well known, isolators are fairly complex devices and, as such, are generally quite expensive. Indeed, then, the precise optical alignment and packaging requirements for such transmitter modules will account for a considerable proportion of the cost for each unit.
In the past, discrete devices have been aligned with other discrete devices or with integrated devices by methods which include some form of active positioning. For example, some systems employ a feedback control loop to monitor light intensity as the devices are being aligned, the devices being fixed in place when the intensity is optimum. Alignment in such a manner has proven to be very difficult and time consuming. In addition to the high costs associated with the required complex components, such techniques necessarily demand very skilled and experienced operators.
While losses within an optical element can be reduced by appropriate component design, it is difficult to factor in the losses associated with coupling between optical elements. Connection losses are inherently variable and this forces the designer to consider the effect of both the maximum and minimum optical signal power levels on the system. Any new method of minimizing coupling losses between active or passive optical elements, minimizing the variability of such losses, or enhancing robustness and manufacturability, will be of tremendous value to the realization of cost-effective optical modules.